A magnetic resonance imaging apparatus is a medical-use diagnostic noninvasive imaging apparatus, utilizing nuclear magnetic resonance phenomenon, which is a phenomenon that hydrogen nucleus (protons) placed in a static magnetic field are resonant with an RF magnetic field at a specific frequency. Since nuclear magnetic resonance signals are changed depending on various physical properties, such as proton density and relaxation time, an image obtained by the MRI can depict various biological information, such as a structure or a composition of living tissues and cell properties.
In recent years, a magnetic susceptibility difference between living tissues receives attention, as one of the physical properties being measurable by the MRI. The magnetic susceptibility is a physical property that represents a degree of magnetic polarization (magnetization) of materials in the static magnetic field. In a living body, there are contained paramagnetic substances such as deoxyhemoglobin in venous blood and iron protein, and diamagnetic substances such as water constituting a large part of the living tissues and calcium serving as a basis of calcification. Creating an image quantitatively from a difference in magnetic susceptibility, i.e., the magnetic susceptibility difference between the living tissues, may be applicable to diagnosis of cerebral ischemia disease, prediction of radiation treatment effect against cancer, and identification of neurodegenerative disease.
A method for creating an image of the magnetic susceptibility difference between living tissues by utilizing the MRI is referred to as quantitative susceptibility mapping (QSM). The QSM is a method of calculating spatial variation in magnetic field caused by the magnetic susceptibility difference between living tissues, from phase information of an MR image being measured, and obtaining a magnetic susceptibility distribution according to a relational expression between the magnetic field and the magnetic susceptibility.
However, a distribution of the magnetic field is obtained by subjecting the magnetic susceptibility distribution to spatial convolution integral. Therefore, obtaining the magnetic susceptibility distribution from the magnetic field distribution is an inverse problem of the convolution integral, and thus a unique solution cannot be obtained.
In general, the least-square method is employed to obtain the magnetic susceptibility distribution from the magnetic field distribution. In this case, an error function is introduced, and a value that minimizes this error function is obtained as a solution. Representative examples of this method are, for example, TKD (Truncated-based K-space Division) (e.g., see Non Patent Document 1), Iterative SWIM (Susceptibility Weighted Imaging and Mapping) (e.g., see Non Patent Document 2 and Patent Document 1), and MEDI (Morphology enabled dipole inversion) method (e.g., see Non Patent Document 3 and Patent Document 2). Here, the TKD is a method of calculating a magnetic susceptibility distribution according to operations on k-space of the magnetic field distribution and of the point dipole magnetic field; the iterative SWIM is a method of merging through an iterative operation, the magnetic susceptibility distribution calculated according to the TKD method, with the magnetic susceptibility distribution where a fine structure is extracted according to thresholding; and the MEDI is a method that utilizes the regularized least-square method.